Cancer of the breast is a major cause of death among the American female population. Effective treatment of this disease is most readily accomplished following early detection of malignant tumors. Major efforts are presently underway to provide mass screening of the population for symptoms of breast tumors. Such screening efforts will require sophisticated, automated equipment to reliably accomplish the detection process.
The x-ray absorption density resolution of present graphic x-ray methods is insufficient to provide reliable early detection of malignant tumors. Research has indicated that the probability of metastasis increases sharply for breast tumors over 1 cm in size. Tumors of this size rarely produce sufficient contrast in a mammogram to be detectable. To produce detectable contrast in graphic mammograms 2-3 cm dimensions are required. Calcium deposits used for inferential detection of tumors in conventional mammography also appear to be associated with tumors of large size. For these reasons, graphic mammography has been relatively ineffective in the detection of this condition.
Most mammographic apparatus in use today in clinics and hospitals require breast compression techniques which are uncomfortable at best and in many cases painful to the patient. In addition, x-rays constitute ionizing radiation which injects a further risk factor into the use of mammographic techniques as most universally employed.
Ultrasound has also been suggested, as in U.S. Pat. No. 4,075,883, which requires that the breast be immersed in a fluid-filled scanning chamber. U.S. Pat. No. 3,973,126 also requires that the breast be immersed in a fluid-filled chamber for an x-ray scanning technique.
In recent times, the use of light and more specifically laser light to non-invasively peer inside the body to reveal the interior structure has been investigated. This technique is called optical imaging. Optical imaging and spectroscopy are key components of optical tomography. Rapid progress over the past decade have brought optical tomography to the brink of clinical usefulness. Optical wavelengths do not penetrate in vivo tissue in a straight line as do x-rays. This phenomena causes the lights to scatter inside the tissue before they emerge out of the scanned sample.
Because x-ray propagation is essentially straight-line, relatively straight forward techniques based on the Radon transform have been devised to produce computed tomography images through use of computer algorithms. Multiple measurements are made through 360.degree. around the scanned object. These measurements, known as projections, are used to back-project the data to create an image representative of the interior of the scanned object.
In optical tomography, mathematical formulas and projection techniques have been devised to perform a reconstruction function somewhat similar to x-ray tomography. Acquiring data through optical measurements of the photons emerging from a scanned object is the first step in the imaging process. Light photon propagation through in vivo breast tissue results in high attenuation of the beam intensity. Methods for detection of low light levels and preserving a wide dynamic measurement range are essential components of a practical computed tomography laser scanner for the breast and other soft tissues. These sophisticated detection techniques are required to acquire the data necessary for meaningful image reconstructions.